1. Field of the Invention
The invention relates to a variable-gain amplifier and related method thereof, and more particularly, to a variable-gain amplifier capable of increasing the bias voltage difference during the input stage and active load of a bias amplifier and related method thereof.
2. Description of the Prior Art
Today, as the popularity of transferring, processing, and exchanging various documents, data, and image information through electronic signals increases, the importance of utilizing various types of signal processing circuits to process electronic signals also increases significantly.
Adequately maintaining the magnitude of the signals in electronic signal systems commonly utilized in communication, media, and medical imaging has become critically important, which has ultimately brought out the development of a variable-gain amplifier. A variable-gain amplifier is an amplifier in nature, in which the amplifier is able to gain input signals and output them thereafter, in which the gaining value can be controlled by another control signal, such as a control voltage. Preferably, the variable-gain amplifier can be utilized to adequately adjust the amplitude of signals, such as forming an automatic-gain control (AGC) circuit to automatically adjust the amplitude of the signals.
Since the gain of the variable-gain amplifier can be controlled by the control signal, the variation of the gain corresponding to the change of the control signal is essentially the gain variability of the variable-gain amplifier. Preferably, a variable-gain amplifier is fabricated to achieve a broader gain range, to have better controllability, such as having a high degree of linearity, and to have adequate resistance against the influence of temperatures during fabrication processes. In general, the gain range of a variable-gain amplifier refers to the range of the variable-gain, or the range between the smallest gain and the largest gain of a variable-gain, and the gain range increases as the range of the variable-gain increases within a fixed variation range of the control signal. Hence, a variable-gain amplifier having a broader gain range is able to significantly increase its gain and accommodate a much larger group of different signals and therefore exhibit a much greater value. The control of the variable-gain on the other hand, can be represented by a characteristic curve, in which the characteristic curve relates to the variation between the gain and a control signal. If the characteristic curve matches a particular curve, such as a straight line close to linearity, it indicates that the variation of the gain can be controlled adequately and a much better gain-variation can be produced. Similar to other electronic circuits, factors such as fabrication process variation and temperature shift in circuit operation may cause the property of each circuit device of variable-gain amplifiers to drift from its original value, and ultimately degrade the actual performance of the variable-gain amplifiers. Hence, a well designed variable-gain amplifier should be able to resist those influential factors caused by the fabrication process and maintain its gain variation under harsh conditions.
However, the conventional variable-gain amplifiers are unable to effectively provide a solution to the situations described above. For instance, despite the fact that some of the conventional variable-gain amplifiers may possess satisfactory controllability and resistance to harsh fabrication environment and temperature variation, the variable-gain amplifiers may only include a small gain range, such that a cascade of a plurality of variable-gain amplifiers must be established to achieve a broader gain range. Unfortunately, the cascade of a plurality of variable-gain amplifiers often requires a large layout area, drains a significantly amount of power, and ultimately compresses the bandwidth of the amplifying signal. Additionally, despite some of the conventional variable-gain amplifiers exhibiting a large gain range, they may have weaker resistance against factors during the fabrication process, such as temperatures fluctuations that ultimately cause the property of the variable-gain to drift from its original value.